Cellular Respiration
Chapter 8
Cellular Respiration
Cellular Respiration – cellular process that
requires oxygen and gives off carbon
dioxide
– Often involves complete breakdown of
glucose to carbon dioxide and water
Oxidation
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
Reduction
Mader; Biology, 9th Ed.
Cellular Respiration
Energy within a glucose molecule is
released slowly so that ATP can be
produced gradually
NAD+ and FAD are oxidation-reduction
enzymes active during cellular respiration
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NAD+ Cycle
NAD+ (nicotinamide
adenine dinucleotide)
– Called a coenzyme of
oxidation-reduction
Oxidize a metabolite by
accepting electrons
Reduce a metabolite by
giving up electrons
– Each NAD+ molecule
used over and over
again
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FAD
FAD (flavin adenine dinucleotide)
– Also a coenzyme of oxidation-reduction
– Sometimes used instead of NAD+
– Accepts two electrons and two hydrogen ions
(H+) to become FADH2
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Glucose Breakdown
4 phases of glucose breakdown:
Glycolysis – glucose is broken down in
cytoplasm to two molecules of pyruvate, some
ATP formed
Transition reaction – pyruvate is oxidized, NADH
is formed, and waste CO2 removed
Citric acid cycle – NADH and FADH2, release of
CO2, and production of additional ATP
Electron transport chain – produces 32/34
molecules of ATP, extracts energy from NADH
and FADH2
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Glucose Breakdown
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Glycolysis
Occurs in cytoplasm, outside mitochondria
Requires initial investment of 2 ATP
– ATP activates glucose to split into PGAL/G3P
Oxidation of PGAL and subsequent
substrates results in 4 high-energy PO4
groups, which synthesize four ATP
– Substrate-Level Phosphorylation
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Substrate-Level
Phosphorylation
Uses an enzyme
to pass a high
energy PO4 from
ADP to ATP
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Glycolysis Net Reaction
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Glycolysis
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Inside the Mitochondria
Pyruvate enters the mitochondria, where it
is converted to 2-carbon acetyl group
– Attached to Coenzyme A to form acetyl-CoA
– Electron picked up (as hydrogen atom) by
NAD+
– CO2 and ATP are transported out of
mitochondria into the cytoplasm
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Mitochondrion Structure and
Function
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Transition Reaction
Preparatory reaction - Connects glycolysis
to the citric acid cycle
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Citric Acid Cycle
Occurs in matrix of mitochondria
Both acetyl (C2) groups received from prep
reaction:
– Acetyl (C2) group transferred to oxaloacetate (C2) to
make citrate (C6)
– Each acetyl oxidized to two CO2 molecules
– Remaining 4 carbons from oxaloacetate converted
back to oxaloacetate (thus “cyclic”)
NADH, FADH2 capture energy rich electrons
ATP formed by substrate-level phosphorylation
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Citric Acid Cycle
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Citric Acid Cycle
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Electron Transport System
Located on cristae of mitochondria (in
Eukaryotes)
– Plasma membrane of aerobic prokaryotes
Made up of a series of electron carriers
3 protein complexes
2 protein mobile carriers
– Complex arrays of protein and cytochromes
Cytochromes are respiratory molecules
Complex carbon rings with metal atoms in center
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Electron Transport System
Electrons enter ETS from NADH and
FADH2
As electrons pass down the electron
transport system, energy is captured and
ATP is produced
– Oxidative phosphorylation – production of
ATP as a result of energy released by ETS
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Electron Transport System
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Organization of Cristae
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ATP Production
H+ gradient created between matrix
and intermembrane space in
mitochondria
– ~10X more H+ in intermembrane space
than within matrix
H+ flows back into matrix by ATP
synthase complex which synthesizes
ATP from ADP
– Chemiosmosis
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Mader; Biology, 9th Ed.
Energy Yield
Net energy yield from glucose
breakdown is 36 or 38 ATP (263/277
kcal energy)
Efficiency of glucose breakdown is
39% (the remaining energy is lost as
heat)
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Fermentation
When oxygen limited:
– Spent hydrogens have no acceptor
– NADH can’t recycle back to NAD+
– Glycolysis stops because NAD+ required
Fermentation:
– Anaerobic pathway
– Can provide rapid burst of ATP
– Provides NAD+ for glycolysis
– NADH combines with pyruvate to yield NAD+
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Fermentation
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Fermentation
Pyruvate reduced by NADH to:
– Lactate
Animals & some bacteria
Cheese & yogurt; sauerkraut
– Ethanol & carbon dioxide
Yeasts
Bread and alcoholic beverages
Allows glycolysis to proceed faster than O2 can be
obtained
– Anaerobic exercise
– Lactic acid accumulates
– Causes cramping and oxygen debt
When O2 restored, lactate broken down to
acetyl-CoA and metabolized
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Fermentation
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Metabolic Pool
Carbohydrates, fats, and proteins in
food can be used as energy sources
Catabolism – degradative reactions
Anabolism – synthetic reactions
– All reactions in cellular respiration are
part of a metabolic pool, and their
substrates can be used for catabolism or
anabolism
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Metabolic
Pool
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Catabolism
Breakdown products enter into respiratory
pathways as intermediates
– Carbohydrates - converted into glucose
Processed via glycolysis…
– Proteins - broken into amino acids (AAs)
Some AAs used to make other proteins
Excess AAs deaminated (NH2 removed) in liver
– Results in poisonous ammonia (NH3)
– Quickly converted to urea
Different R-groups from AAs processed differently
Fragments enter respiratory pathways at many
different points
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Anabolism
Intermediates from respiratory pathways can be used for
anabolism
– Carbs
Start with acetyl-CoA
Basically reverses glycolysis (but different pathway)
– Fats
G3P converted to glycerol
Acetyls connected in pairs to form fatty acids
Note – dietary carbohydrate RARELY converted to fat in humans!
– Proteins - Made up of combinations of 20 different amino acids
Some amino acids (11) can be synthesized from respiratory
intermediates
– organic acids in citric acid cycle can make amino acids
– Add NH2 – transamination
However, other amino acids (9) cannot be synthesized by humans
– Essential amino acids
– Must be present in diet
Mader; Biology, 9th Ed.
Mader; Biology, 9th Ed.